Discussion

- Morphometry of Vertebral Pedicles: a Comprehensive Anatomical Study in the Lumbar Region

Chaynes et al. reported a
cadaveric study on vertebral pedicle anatomy in relation to pedicle
screw fixation. They reported that the pedicular height in spine
gradually increases from T1 to L5. On the contrary in the present study
the vertical height (v) in bone specimens increased from LI to L2,
decreased from L2 to L3, increased from L3 to L4 and markedly increased
from L4 to L5. In the present work, the minimum horizontal diameter (d)
in lumbar spine increased from LI to L5, whereas in the study reported
by Chaynes et al. (2001), the pedicular width was greatest at
the three junctional regions of the spine. Linear measurements of both
vertebral pedicles were made in ten complete lumbar cadaveric spines
using calipers in study reported by Chaynes et al. , whereas
our study report the same was based on measurements of bone specimens
of fully ossified 200 male and 200 female lumbar vertebrae greater than
20 years of age; hence the aforementioned differences may be one of the
factors responsible for the different observations.

Analyses of the observations with
prior works by other authors reveal many interesting facts. In previous
reports by Amonoo- Koufi and Singel et al. , values for left
and right pedicles of the lumbar vertebra were nearly the same, on the
contrary in our study the results were different and hence presented
separately. Singel et al. reported that nearly all the
measurements were greater in females than the males, whereas in our
report all the mean values of the parameters observed in male subjects
scored significantly higher and this can be justified as they have to
support greater upper body weight. The anteroposterior length from body
of the vertebra to the transverse process along the long axis was not
included in majority of the previous works, although it is important
for screw fixation techniques. These values were maximum at L2 level
and minimum at L5 level in both left and right pedicle in both the
sexes. Short stature of average Indian population compared to the
western world can be correlated for the decreased value of all the
parameters bilaterally from LI to L5 in both the sexes. Greater
vertical height (v) values of L2 vertebra than LI and L3 is due to the
closer location of the first lumbar pedicle to the transitional
junction of thoracolumbar spine, and is again the site of complex
zygapophyseal joint. The brunt of this force transmission should then
be on L2 vertebra which has adapted itself resulting in increased
pedicle vertical height. This stable position makes the L2 vertebra
less prone to dislocate as compared to LI transition vertebra. All the
explanations given by previous authors (Amonoo-Kuofi; Coles et al. 1998; Fidler, 1988; Krenz & Troup, 1973; Postachini & Cinotti, 1992; Singel et al) regarding
the greater vertical height of LI pedicle will finally apply to the L2
pedicle. The uniform increase in minimum horizontal diameter from L1 to
L5 bilaterally in both males and females is similar to observations and
explanations given by the earlier authors and is primarily related to
the role of weight transmission.

Age related variations observed from
X-rays concluded some facts which could not be assessed from direct
gross measurements. The maximum horizontal diameter (t) of both right
and left pedicles from radiographs of female subjects were
significantly greater than male counterpart at all vertebral segments
from LI to L5 in adolescent age group (10-20 years), this finding was
contrary to the rest of all the age groups (>20 years), where it was
significantly greater in males. Aforementioned observations can best be
correlated with early pubertal growth spurts in females than males,
which reflected as different trend in 10-20 years age group. Same
explanation applies to the maximum vertical height (h), though the
differences were not statistically significant, inferring that this
parameter was relatively less affected by pubertal changes in both the
sexes. Similar observations were reported by Amonoo- Kuofi, though his
study was different as there were no differences between right and left
pedicles measurement and the gender difference were not significant in
majority of elder (> 50 years) age groups. He related the larger
vertical diameter of the pedicle of the 1 st lumbar vertebra (in both
males and females) as compared with the vertical diameters of 2nd and
3rd lumbar pedicles by the weight bearing functions and concluded that
vertical diameter with its larger dimension and wider variations with
age, contributes more to weight- bearing function than the horizontal
diameter. He opined that after the 5th decade of life the horizontal
and vertical diameters of pedicles in females showed a tendency to
increase, while male diameters decreased (1995). On the contrary in the
present study the aforementioned diameters in elder (>50 years) age
group showed a tendency to decrease in both the sexes. Hence it can be
hypothesized that age related osteoporosis along with wear and tear
processes is responsible for decreasing diameters in the X-rays.

What are the factors contributing to
the measured differences between specimens and the X-rays in the
present study? Following possibilities can be hypothesized:

1.

Partial
volume effects are responsible for this phenomenon which resulted in
greater values in the radiological study than the direct specimen
observations.

2.

In
plain radiographs, magnification factor can only be roughly estimated,
besides X-rays measurements had the common limitation of the two
dimensional structures.

3.

Bone
specimen observations had the usual benefit of three dimensional
structures; on the other hand, the fact that some dry bone properties
differ from the in vivo bones was the main limitation.